"The question is, is it too late to plant grass? "

It's not ideal, but it's certainly not too late to plant grass seed in
Philly. Fescue will germinate in a week, blue grass will take 2-3
weeks. Assuming normal weather, meaning it doesn't get cold a lot
sooner than usual, you will be OK. It's been so hot and dry here in
central NJ that I did some seeding I needed to do only last Fri, right
before the big rain.

And there's not much downside, since you;re just patching some areas.
I've seeded full lawns in late Oct and had good results. The biggest
concern there is that it won't have enough time to fill in real well
till spring, so if you're doing a whole lawn, it may not look great for
the winter. But that shouldn't be a problem for a few spots.

If you want a sure method, get some sod, which is an option for small
patch areas.